Plasma arc reactor for the production of fine powders

ABSTRACT

A plasma arc reactor and process for producing a powder from a solid feed material, for example aluminium, is provided. The reactor comprises: (a) a first electrode ( 5 ), (b) a second electrode ( 10 ) which is adapted to be spaced apart from the first electrode by a distance sufficient to achieve a plasma arc therebetween, (c) means for introducing a plasma gas into the space between the first and second electrodes, (d) means for generating a plasma arc in the space between the first and second electrodes, wherein the first electrode has a channel ( 7 ) running therethrough, an outlet of the channel exiting into the space between the first and second electrodes, and wherein means are provided for feeding solid material ( 20 ) through the channel to exit therefrom via the outlet into the space between the first and second electrodes.

The present invention relates to an apparatus and process for theproduction of powders. In particular, a plasma arc reactor is providedwhich may be used in a plasma evaporation process to produce sub-micronor nano-metric (i.e. nano-sized) aluminium powders.

Metal and ceramic powders are used in sintering processes in metallurgyand in catalysis in the chemical industry. The powders may be used inthe manufacture of structural components, magnetic films, chemicalcoatings, oil additives, propellant additives and also in explosives.

The present invention provides a plasma arc reactor for producing apowder from a solid feed material, the reactor comprising:

-   (a) a first electrode,-   (b) a second electrode which is adapted to be spaced apart from the    first electrode by a distance sufficient to achieve a plasma arc    therebetween,-   (c) means for introducing a plasma gas into the space between the    first and second electrodes,-   (d) means for generating a plasma arc in the space between the first    and second electrodes,    wherein the first electrode has a channel running therethrough, an    outlet of the channel exiting into the space between the first and    second electrodes, and wherein means are provided for feeding solid    material into and through the channel to exit therefrom via the    outlet into the space between the first and second electrodes.

The term electrode as used herein is intended to encompass a plasmatorch.

The first electrode is preferably moveable with respect to the secondelectrode from a first position at which an arc portion thereof contactswith an arc portion of the second electrode to a second position atwhich said arc portions are spaced apart from each other by a distancesufficient to achieve a plasma arc therebetween. This is advantageousbecause contacting the first and second electrodes assists in startingthe plasma arc. It will be appreciated that by the term arc portion ismeant those regions or points on the surfaces of the first and secondelectrodes between which a plasma arc may be generated.

The first electrode may preferably take the form of a hollow elongatemember whose inner surface defines a closed channel (equivalent to abore or passageway). The elongate member terminates at an arc tip,which, in use, will oppose an arc portion of the second electrode. Theoutlet of the closed channel is disposed at or adjacent to the arc tip.In this case, the first electrode may be in the form of a hollow rod,cylinder or tube. The first electrode may be formed initially as ahollow object. Alternatively, the first electrode may be formed as asolid object, to subsequently have a bore or passageway formed therein.If the outlet is disposed at the arc tip, then it will be appreciatedthat the end surface of the elongate member will define both the arc tipof the electrode and the outlet of the closed channel. the firstelectrode will typically act as the cathode.

The second electrode, which acts a counter electrode, may take anyappropriate form to enable a plasma arc to be generated between it andthe first electrode. The second electrode may simply have asubstantially planar arc portion. For example, the second electrode maybe disposed as a planar substrate on the bottom wall of the plasmareactor.

The arc portions of the first and/or second electrodes will generally beformed from carbon, preferably graphite.

The plasma reactor may be provided in the form of a graphite linedvessel or a graphite crucible, in which a portion thereof acts as thesecond electrode. Accordingly, the second electrode may be integrallyformed with the reactor vessel.

In a preferred embodiment, part or all of the interior surface of theplasma arc reaction chamber constitutes the second electrode. Thechamber may be a graphite reaction chamber or a graphite lined reactionchamber.

It is also preferable to make the second electrode the anode so that, inuse, metal ions are electrostatically repelled from it.

Neither the first electrode nor the second electrode needs to beearthed.

The plasma arc reactor advantageously further comprises cooling meansfor cooling and condensing solid material which has been vaporised inthe plasma arc generated between the first and second electrodes. Thecooling means preferably comprises a source of a cooling gas.

The second electrode preferably comprises a graphite vessel having asurface adapted to direct vaporised material downstream to a coolingzone to be cooled, in use, by a cooling gas.

A collection zone may be provided downstream of the cooling zone forcollecting a powder of the condensed vaporised material. The collectionzone may comprise a filter cloth which separates the powder particulatefrom the gas stream. The filter cloth is preferably mounted on anearthed cage to prevent electrostatic charge build up. The powder maythen be collected from the filter cloth, preferably in a controlledatmosphere zone. The resulting powder product is preferably then sealed,in inert gas, in a container at a pressure above atmospheric pressure.

The channel in the first electrode is advantageously adapted toadditionally introduce the plasma gas into the space between the firstand second electrodes. Thus, the solid feed material and plasma gas maytravel through a common channel and exit the electrode via a commonoutlet into the space between the first and second electrodes.

The means for generating a plasma arc in the space between the first andsecond electrodes will generally comprise a DC or AC power source.

If desired, one or more additional electrodes, also having a materialfeed channel therein, may be used to enable different materials to beco-fed into a single plasma reactor. A common counter electrode may beused or, alternatively, separate counter electrodes may be provided,each opposing an electrode with a channel therein. Common or separatepower supplies may be used, although separate power supplies arepreferred since this allows different evaporation rates for differentmaterials.

The apparatus according to the present invention may operate withoutusing any water-cooled elements inside the plasma reactor and may alsoallow replenishment of solid feed material without stopping the reactor.Water cooling may result in thermal shock and material fracture. Theremay also be undesirable reactions between water vapour and the materialbeing processed.

The apparatus according to the present invention may further comprisemeans for conveying solid feed material to the first electrode. If thesolid feed material is in the form of a wire, then the apparatuspreferably comprises a supply of wire. For example, the apparatus maycomprise a container or holder for the wire, preferably a coil or reel.Means are also preferably provided for conveying the wire from thesupply of wire to the first electrode, where the wire is fed into thechannel. this may be achieved, for example, by the use of an electricmotor.

The present invention also provides a process for producing a powderfrom a solid feed material, which process comprises:

-   (i) providing a plasma arc reactor as herein described,-   (ii) introducing a plasma gas into the space between the first and    second electrodes,-   (iii) generating a plasma arc in the space between the first and    second electrodes,-   (iv) feeding solid material through the channel to exit via the    outlet thereof into the plasma arc, whereby the solid feed material    is vaporised,-   (v) cooling the vaporised material to condense a powder, and-   (vi) collecting the powder.

The process according to the present invention may be considered agas/vapour phase condensation process. In such a process, a plasma isgenerated to vaporise the solid feed material and material fragmentationoccurs in the vapour phase. The vapour is subsequently cooled andtransforms to a solid phase particulate.

The solid feed material will generally comprise or consist of a metal,for example aluminium, nickel or tungsten, including alloys that includeone or more of these metals. Aluminium and aluminium alloys arepreferred. The solid feed material may be provided in any suitable formwhich allows it to be fed into and through the channel to exit therefrominto the space between the electrodes. For example, the material may bein the form of a wire, fibres and/or a particulate. The solid feedmaterial does not need to be provided in a secondary supporting phase,such as a liquid carrier.

The solid feed material is preferably provided in the form of acontinuous wire. This is advantageous because it has been found thatproviding the solid feed material in the form of a wire assists indelivering the material to the plasma zone and into the plasma core.

The plasma gas will generally comprise or consist of an inert gas, forexample helium and/or argon.

The plasma gas is advantageously injected into the channel in the firstelectrode to exit therefrom into the space between the first and secondelectrodes. In this case, the plasma gas and solid material preferablyexit the first electrode via a common outlet. The plasma gas and solidmaterial may be fed into the channel in the first electrode via a commoninlet or, alternatively, via separate inlets. During operation, theplasma gas and solid material will be co-fed into the channel.

The volume flow rate of the plasma gas is preferably monitored tooptimise material-to-plasma heat transfer characteristics and toencourage the material to partition towards the vapour phase.

At least some cooling of the vaporised material may be achieved using aninert gas stream, for example argon and/or helium. Alternatively, or incombination with the use of an inert gas, a reactive gas stream may beused. The use of a reactive gas enables oxide and nitride powders to beproduced. For example, using air to cool the vaporised material canresult in the production of oxide powders, such as aluminium oxidepowders. Similarly, using a reactive gas comprising, for example,ammonia can result in the production of nitride powders, such asaluminium nitride powders. The cooling gas may be recycled viawater-cooled conditioning chamber.

The surface of the powder may be oxidised using a passivating gasstream. This is particularly advantageous when the material is aluminiumor aluminium-based. The passivating gas may comprise anoxygen-containing gas, and a particularly preferred gas comprises from95 to 99 vol. % of an inert gas, such as helium and/or argon, and from 1to 5 vol. % of oxygen, more preferably approximately 98 vol. % of theinert gas(es) and approximately 2 vol. % of oxygen. Such a gas mixturehas been found to produce particularly good results for aluminium andaluminium-based materials. The passivating gases are preferably premixedto avoid local gas phase enrichment and the possibility of explosions.The (inert) cooling gas may be recycled and subsequently diluted withoxygen at a rate of typically 1 NM³/hour to provide the passivating gasstream. The aluminium acts as a getter for the oxygen and reacts with itwith the result that the partial pressure inside the chamber falls. Ifthe pressure in the chamber is monitored, then a subsequent rise in thepartial pressure indicates that the surface of the aluminium powder hasbeen substantially fully passivated. The reactivity of some ultra-finepowders presents an operational risk if there is a likely-hood ofcontact with, for example, water and/or air. The passivation stagerenders the powdered material more suitable for transporting.

For aluminium for certain applications, it is preferable thatsubstantially no oxidation occurs in the plasma. It also preferable thatcooling of the vaporised material is achieved using an inert gas stream,for example argon and/or helium. Accordingly, the passivation stagepreferably occurs only after the powder has been cooled. In a preferredembodiment, the solid feed material, for example aluminium wire, is fedto the core of the plasma where it is vaporised. The metal vapour isthen conveyed to a separate quenching region where it is quenched in aninert gas stream and transforms to a solidified powder. This solidpowder is then exposed to oxygen under low temperature oxidationconditions so that the oxide grows to a limiting thickness and then selfregulates, i.e. the oxide inhibits further oxidation. This oxygenexposure/reaction process occurs away from the plasma core.

The process according to the present invention may be used to produce apowdered material, such as aluminium, substantially all of the particlesof which having a diameter of less than 200 nm. Preferably the averageparticle diameter lies in the range of from 50 to 150 nm, morepreferably from 80 to 120 nm, still more preferably from 90 to 110 nm.

Specific surface area analysis has shown that the process according tothe present invention may be used to produce a powdered material, suchas aluminium, which has a specific surface area typically in the rangeof from 15 to 40 m²g⁻¹, more typically in the range of from 25 to 30m²g⁻¹.

It will be appreciated that the processing conditions, such as materialand gas feed rates, temperature and pressure, will need to be tailoredto the particular material to be processed and the desired size of theparticles in the final powder.

Preferably, part or all of the interior surface of the reaction vesselconstitutes the second electrode. The second electrode is preferably theanode and the first electrode is preferably the cathode. For certainapplications, the first and/or the second electrodes are preferablyformed from a material that does not react with the feed material at thetemperature involved.

Both the first and second electrodes are preferably formed of a carbonmaterial, more preferably graphite. Accordingly, the reaction vessel maybe a graphite reaction chamber or a graphite lined reaction chamber,which constitutes the second electrode.

It is generally preferable to pre-heat the reactor before vaporising thesolid feed material. The reactor may be preheated to a temperature of upto typically 2500° C., more typically from 500° C. to 2500° C. For analuminium feed material, the reactor is preferably preheated to atemperature of from 2000° C. to 2500° C., more preferably from 2200° C.to 2500° C., still more preferably from 2300° C. to 2500° C. Pre-heatingmay be achieved by any suitable means, although it is preferablyachieved using the plasma arc. Preferably, substantially all of theinterior of the reaction vessel is pre-heated.

The rate at which the solid feed material is fed into the channel in thefirst electrode will affect the product yield and powder size. Whenusing aluminium wire, a feed rate of from 1 to 5 kg/hour has been used,more typically approximately 2 kg/hour. The aluminium wire is typically1 to 10 mm gauge, more typically 1 to 5 mm.

The inert plasma gas, for example helium, may also be injected throughthe channel in the first electrode at a rate of from 2.4 to 6 Nm³/h,more typically approximately 3 Nm³/hour.

If a DC power supply is used to generate the plasma arc, then the DCamperage will generally be set at a value in the range of from 400 to800 A. Typical DC Electrical characteristics are of the order of 800 Aand between 30 to 40 V with a plasma arc column length of between 60 mmand 70 mm.

The process and the plasma arc reactor according to the presentinvention are typically operated at above atmospheric pressure, moretypically in excess of 750 mm of water above atmospheric pressure. Thisprevents or assists in preventing the ingress of atmospheric oxygen intothe plasma zone, which may result in an undesirable chemical reaction.When the feed material is aluminium, it is preferable to operate theplasma arc reactor above atmospheric pressure, typically up to 45 inWG(inches water is gauge), more typically from 15 to 35 inWG. Operating ata pressure above atmospheric pressure also has the advantage that itresults in a higher yield of particulate material.

If a cooling gas, preferably an inert gas such as argon or helium isused to cool and condense the vaporised material, a flow rate of from 60to 120 Nm³/h has been found to result in an aluminium powder in whichmost, if not substantially all, of the particles have a diameter of lessthan 200 nm in diameter (more typically ≦100 nm). After cooling, the gasand particulate temperature will typically be from 300 to 350° C.

For an aluminium feed material, the process according to the presentinvention may be used to produce a powdered material having acomposition based on a mixture of aluminium metal and aluminium oxide.This is thought to arise with the oxygen addition made to the materialduring processing under low temperature oxidation conditions.Accordingly, the present invention also provides a particulate materialcomprising particles having a core comprising or consisting essentiallyof aluminium and a surface layer comprising or consisting essentially ofaluminium oxide, which particulate material is obtainable by a processas herein described.

Substantially only the surface of the particles oxidise and surfacespecific analysis has shown that the oxide component of the powder isassociated generally with the surface and the oxide layer is typicallyless than approximately 10 nm in thickness, more typically less thanapproximately 5 nm in thickness. Hence, such a material can be describedas discreetly encapsulated. Substantially all of the particles of theoxide coated aluminium have a diameter of less than 200 nm and theaverage particle diameter will typically lie in the range of from 50 to150 nm, more typically from 80 to 120 nm, still more typically from 90to 110 nm. The specific surface area of the oxide coated aluminiumparticles will typically be in the range of from 15 to 40 m²g⁻¹, moretypically in the range of from 25 to 30 m²g⁻¹.

Examination of the particulate using TEM and electron diffractionindicates that the aluminium particles are essentially single crystal,i.e. mono-crystalline.

The present invention will now be described further, by way of example,with reference to the accompanying drawings in which:

FIG. 1 shows one embodiment of an electrode configuration which may beused in a plasma arc reactor according to the present invention;

FIG. 2 provides a flow diagram of a process according to the presentinvention;

FIGS. 3( a) and (b) are secondary electron micrographs of aluminiumpowders made by the process according to the present application(magnification: ×100,000 (a) and ×200,000 (b));

FIG. 4 is a graph showing the variation of specific surface area of anideal nanometric aluminium powder with particle diameter;

FIG. 5 is a graph showing the variation of oxide content of an idealnanometric aluminium powder with particle diameter;

FIG. 6 is a graph showing primary (1^(st) heat) DSC analysis for analuminium sample;

FIG. 7 is a graph showing secondary (2^(nd) heat) DSC analysis for analuminium sample; and

FIG. 8 is a survey spectrum of nanometric Al powder analysed by XPS.

In FIG. 1, a first electrode 5 is provided in the form of a cylindricalgraphite rod which terminates at an arc tip 6. If desired, the upperportion of the graphite electrode 5 may be replaced with copper. Theelectrode 5 has a central bore formed therein which extends along thelength of the electrode 5. The surface of the bore defines a closedchannel 7 (or passageway) having an inlet 8 at one end and an outlet 9disposed at the arc tip 6.

A second counter electrode 10 is provided as part of a graphite-linedreactor vessel (13) (see FIGS. 1 and 2). Only an arc portion 11 on theinterior surface of the bottom wall 12 of the vessel 13 is shown inFIG. 1. The whole of the vessel 13 is shown in FIG. 2 and it can be seenthat the counter electrode forms an integral part of the reactor vessel13. The arc portion 11 of the second electrode 10 opposes the arc tip 6of the first electrode 5.

The first 5 and second 10 electrodes are connected to a DC power supply15. The first electrode 5 is the cathode and the second electrode is theanode 10, although it will be appreciated that the polarities may bereversed.

The first electrode 5 is moveable with respect to the second electrode10 and hence may be lowered to contact at the arc tip 6 thereof with thearc portion 11 of the second electrode 10 to complete the electricalcircuit. The DC amperage from power supply 15 will generally be set at avalue from 400 to 800 A. By raising the first electrode 5, a DC plasmaarc can be established between the arc tip 6 of the first electrode 5and the arc portion 11 of the second electrode 10.

A solid feed material, for example aluminium wire 20, can be fed intothe inlet 8, to pass down the channel 7, out the outlet 9 and into thespace between the arc tip 6 of the first electrode 5 and the arc portion11 of the second electrode 10. An inert plasma gas 25, such as argonand/or helium, may similarly be injected through the channel 7, via theinlet 8, to exit the first electrode 5 at outlet 9. Accordingly, boththe aluminium wire 20 and the plasma gas 25 may enter the firstelectrode 5 via a common inlet 8 and exit the electrode 5 via a commonoutlet 9 at the arc tip 6.

The wire 20 may be stored by conventional means on a coil or reel andfed by a multi-speed motor into inlet 8. The plasma gas 25 may be storedby conventional means in a gas tank, and controlled injection into theinlet may be achieved by the use of a valve. Accordingly, the feed ratesof both the wire and the plasma gas may be controlled.

In use, the graphite-lined vessel 10 is preheated to a temperature of atleast about 2000° C. (typically approximately 2200° C. to 2300° C.)using the plasma arc. This entails injecting an inert plasma gas 25through channel 7 in the first electrode 5 and switching on the powersupply 15.

The reactor is typically operated in excess of 750 mm of water aboveatmospheric pressure.

Once the reactor has been pre-heated, aluminium wire 20 is then fed intothe inlet 8 of channel 7 in the first electrode 5 at a rate of typically2 kg/hour. Inert plasma gas is also injected through channel 7,typically at a rate of from 2.4 and 6 Nm³/h, more typicallyapproximately 3 Nm³/hour.

Typical DC electrical characteristics are of the order of 800 A and from30 to 40 V with a plasma arc column length of from 60 mm and 70 mm.

In this manner, the aluminium wire 20 is vaporised in the hot plasma gas(step A in FIG. 2). The wire 20 and plasma gas 25 are continually fedinto the channel 7 of the first electrode 7 as the wire 20 is vaporisedin the plasma arc. Eventually a steady-state will be achieved. It willbe appreciated that the feed rates of the wire 20 and/or gas 25 may beadjusted during processing.

The vaporised aluminium and plasma hot gas exits the reactor vesselunder the influence of the gas being injected through the channel 7 inthe first electrode 5. The vaporised aluminium is then quenched in acooling zone 30 using an inert cooling gas stream, such as argon orhelium, to condense a sub-micron powder of aluminium (step B in FIG. 2).The flow rate of the cooling gas stream is typically from 60 to 120Nm³/h, and the particles of the aluminium powder are typically ≦200 nmin diameter (more typically ≦100 nm). After the inert gas quench, thegas and particulate temperature is typically from 300 to 350° C.

If desired, a passivation step may next be carried out in a passivationzone 35 downstream of the cooling zone 30 (step C in FIG. 2). This maybe achieved in a number of ways. The cooling gas may be recycled to awater-cooled conditioning chamber for further cooling, and then injectedback into the apparatus, together with up to 5 vol. % of oxygen tocontact with the powder. Typically, the oxygen is introduced at a rateof approximately 1 Nm³/h. Alternatively, a separate source of thepassivation gas may be used. The temperature during the passivation stepis typically in the range of from 100 to 200° C.

After the passivation step, the powder particulate and gas stream passto a collection zone 40 which contains a filter cloth (not shown) toseparate the particulate from the gas (see step D in FIG. 2). The filtercloth is preferably mounted on an earthed cage to prevent electrostaticcharge build up. The gas may be recycled.

The powder may then be collected from the filter cloth, preferably in acontrolled atmosphere zone. The resulting powder product is preferablythen sealed, in inert gas, in a container at a pressure aboveatmospheric pressure.

If desired, one or more additional electrodes having a channel thereinmay be used to co-feed different metals into a single plasma vessel toproduce, for example, alloy powders, sub-micron and nano-sized mixtures,oxides and nitrides. A common counter electrode may be used or,alternatively, separate counter electrodes may be provided, eachopposing an electrode with a channel therein. Common or separate powersupplies may be used, although separate power supplies are preferredsince this allows for different evaporation rates for different metals.

EXAMPLE

This example relates to the production of an nano-metric aluminiumpowder using atmospheric DC plasma technology, which is a clean,controllable and directional heat source. Aluminium powders may be usedin sintering processes in metallurgy and in catalysis in the chemicalindustry. The powders may be used in the manufacture of structuralcomponents, magnetic films, chemical coatings, oil additives, propellantadditives and also in explosives.

The process utilises the mechanism of gas phase condensation. Theprocess offers the advantage of high throughput (Kg/hr) under mixedinert gas process conditions, followed by controllable materialpassivation during pneumatic conveyance and dispersion above atmosphericpressure. The material is produced, cooled, passivated (i.e. surfaceoxidised under low temperature condition), collected and packaged in ahighly monitored and automated manner.

The original feed wire (precursor) used in the process is a wroughtalloy with the designation 1050A, ASTM=ER1100, DIN=S-AL 9915. This wirehas a nominal composition of 99.5 wt % Al, the main impurities are Siand Fe at a maximum of 0.25 wt % and 0.40 wt % respectively.

The aluminium and aluminium oxide content cannot be determined directly,so a quantitative elemental analysis of major powder components wasundertaken. The calculation assumed all oxygen was combined as aluminiumoxide, having the stochiometry Al₂O₃. A pre-calibrated Leco TC436 oxygenand nitrogen analyser was used to determine oxygen content. Apre-calibrated Leco CS344 carbon and sulphur analyser was used forcarbon analysis. Energy dispersive X-ray fluorescence spectroscopy(EDXRF) was used to check powder for high levels of contamination. AnARL 3410 inductively coupled plasma atomic emission spectrometer(ICPAES) was used to quantitatively analyse the solutions for high levelof contaminant identified by EDXRF.

The EDXRF analysis showed significant levels of calcium, although othercontaminants were found at very low levels, for example Fe, Na, Zn andGa. Hence the quantitative analysis focussed on O, C and Ca. The Alcontent can be assumed to make up the majority of the remaining powderafter subtraction of the alumina, calcium and carbon content. The carboncontent was assumed as elemental due to the insoluble residue left inthe container during ICPAES analysis. The analysis results are shown inTable 1

TABLE 1 Combined Material Analysis Results Specimen C Ca O* CalculatedCalculated ID wt % wt % wt % wt % Al₂O₃ wt % Al 6 AL 2.48 0.17 14.9 6 AL2.41 0.17 15.4 6 AL 16.3 Mean 2.44 0.17 15.5 33 64.4 *Oxygen ispurposely added to the system under low temperature oxidation conditions

Aluminium powder samples have been examined by scanning electronmicroscopy (SEM) using a Leica Cambridge S360 instrument. Electronmicro-graphs were prepared to show the size and shape of the particles.Quantitative energy dispersive (ED) X-ray analysis was carried out todetermine the elements present in the sample using a PGT IMIX X-rayanalysis system attached to the SEM.

Secondary electron detection was used to give topographic texturedimages of the aluminium powder particulate and associated agglomerates.At low magnification (×350 magnification) the powder product wasobserved to have agglomerated. The size of the agglomerates ranged fromless than 5 μm to more than 200 μm. At higher magnification (×20,000 and50,000 magnification) the individual particles could be imaged. Theirsize (i.e. largest dimension) was observed to be approximately 100 nm±50nm, however the particles still appeared to be clumped together. Theseagglomerates were determined to be made up of these finer particles. Theshape of the particles seemed to be irregular either spherical or oval.The shape of the individual particles and the process of agglomerationare thought to occur to minimise the excess surface free energyassociated with such a finely divided material. Two secondary electronmicrographs are shown in FIGS. 3( a) and (b).

Transmission electron microscopy (TEM) has shown that the particlestypically have a generally spherical morphology. Corresponding electrondiffraction work indicates that the particles are typically essentiallymono-crystalline.

The specific surface area (SSA) was determined by nitrogen absorption,using the continuous flow method, described in BS 4359 Part 1. The SSAwas shown to lie in the range of from 25 to 30 m²g¹. FIG. 4 shows thevariation of specific surface area with particle size for an idealchemically pure, spherical aluminium powder. FIG. 4 indicates that amean particle size of 90 nm is consistent with a specific surface areaof from 25 to 30 m²g⁻¹. So the SEM images show consistency with the SSAanalysis.

The fraction of oxide in the powder will shift unfavourably as thepowder particulate size becomes smaller, i.e. the proportion of oxidewill increase relative to that of the metal. This trend is graphicallyrepresented in FIG. 5, here a uniform oxide layer of 4.5 nm thickness isassumed. This represents the diffusion limited adherent, coherent anduniform oxide film associated with aluminium material exposed to anoxygen rich atmosphere under low temperature conditions.

Again the compositional analysis indicated an oxide content of 33 wt %,this gives rise to an implied particle size of from 90 to 100 nm. Thisagain being consistent with the SSA analysis and SEM images.

Thermal analysis was carried out using a Differential ScanningCalorimetry (DSC). The instrument was initially checked for temperatureand energy calibration using a traceable indium standard. The sample washeated to 750° C. at a heating rate of 10° C. min⁻¹ under air flowing at5 ml min⁻¹. The DSC spectrum shows an exothermic (energy is released)peak with an extrapolated onset temperature of 538° C. The peak range isfrom 538 to 620° C., with the peak maximum at 590° C. After initialheating the sample was cooled and reheated under the same conditions andno exotherm was observed. This indicated a complete and irreversiblechemical reaction, i.e. oxidation of aluminium. This is graphicallyrepresented in the FIGS. 6 and 7.

The technique of X-ray Photoelectron Spectroscopy (XPS) is surfacesensitive and the outermost 2 to 3 layers of a material (i.e. the top 1nm) is typically analysed. This gives both compositional and chemicalinformation. For example, XPS can distinguish between Al as a bulk metaland Al associated with an oxide Al₂O₃. The survey spectrum showed thepresence of the following species, Table 2:

TABLE 2 Peak Component Assignment 284.7 Cls Environmental CarbonContamination 72.2 Al2p Metal 74.2 Al2p Al₂O₃ 531.6 Ols Al₂O₃ 1071.5Nals Na₂CO₃ 2061.3 Na Auger Parameter Na₂CO₃ 289.4 Cls Na₂CO₃

The survey spectrum is provided in FIG. 8 and shows the presence ofcarbon (19 atomic %), oxygen (50 atomic %), aluminium (27 atomic %),nitrogen (0.6 atomic %), sodium (3.3 atomic %) and calcium (0.7 atomic%). These values were calculated using published sensitivity factors(Briggs and Seah 1990). Detailed spectra were taken off the main peak toprovide chemical information in the form of binding energies, we did nottake into account factors such as morphology, topography andheterogeneity. The carbon peak was used to calibrate the spectrum, i.e.adventitious carbon contamination (environmental contamination), bindingenergy 287.4 eV. The compositional information relates to the outer 2 to3 layers of the particulate material and, accordingly, should not beinterpreted as the overall bulk composition of the material.

Al2p peak showed two superimposed components due to the metal and thenative oxide with binding energies of 72.1 eV and 74.1 eV respectively.The fact that the aluminium metal associated with the interior of theparticulate could be detected, i.e. the substrate metal, through theoxide indicates a thin overlayer of less than 2 to 3 monon-layers(Crystallography: Corundum has a rhombohedral crystal system wherea=b=c=12.98 Å). The carbon peak was observed to be made up of twocomponents, i.e. environmental contamination and carbide. The carboncould not be categorically associated with any one of the metal speciesdetected. The sodium is probably present as carbonate (Na₂CO₃).

It is possible to estimate the thickness of the monolayer using the DeBeers-Lambert equations and associated assumptions.

De Beers-Lambert Equation, Version 1I ^(ox) =I _(o) ^(ox)[1−exp(−d/λ sin θ)]  (1)

De Beers-Lambert Equation, Version 2I ^(element) =I _(o) ^(element)[exp(−d/λ sin θ)]  (2)

-   -   where λ is the inelastic electron mean free path        λ=0.05(KE)^(0.5) nm=0.05(1486.6−73)^(0.5)=1.8799 nm (KE=ejected        electron kinetic energy)

If the oxide is native to the metallic material element then I₀ and λare approximately the same. Hence, by dividing equation 1 by equation 2,an equation relating the relative Al signal intensities to oxide layerthickness may be obtained:I ^(ox) /I ^(element)=exp(d/λ sin θ)−1  (3)

The assumptions associated with the use of this equation are as follows:

-   (i) the surfaces are flat;-   (ii) the oxide layer is uniform in thickness;-   (iii) the layer is continuous; and-   (iv) the surfaces are planar.

The outcome of this calculation is that the oxide layer is approximately2 to 3 nm in thickness, which is consistent with the compositionalanalysis, SSA analysis and SEM images. The variability being associatedwith the inaccuracy of the assumptions made in calculation. Thiscalculation is very inaccurate, however the technique will only analysethe uppermost nm of sample as a maximum depth. This means that as aparent metal signal is observed in the survey spectrum, the oxidethickness must be less than 5 nm, this being a definite statementassociated with the nature of the characterising radiation.

The particulate material according to the present invention has thefollowing characteristics:

-   1. In composition the material is observed to be a mixture of    aluminium metal and aluminium oxide, which is consistent with the    oxygen addition made to the material during processing under low    temperature oxidation conditions, i.e. substantially only the    surface oxidises.-   2. Imaging indicates that the material is formed with a fine    spherical particulate morphology of from 70 to 130 nm in average    particle diameter, (more typically from 80 to 120 nm, still more    typically approximately 100 nm). This justifies the classification    as a nanomaterial.-   3. The particles are agglomerated by which is meant an assembly of    particles held together by weak forces that can be overcome by    suitable means, for example sonication.-   4. Specific surface area analysis has shown that the material has a    specific surface area typically in the range of from 15 to 40 m²g⁻¹,    more typically in the range of from 25 to 30 m²g⁻¹. This correlates    typically to a particle size of from 75 to 95 nm.-   5. Thermal analysis has shown complete and irreversible chemical    reaction takes place in air at 550 to 650° C. This being consistent    with thermally driven oxidation.-   6. Surface specific analysis has shown that the oxide component of    the powder is associated with the surface and the layer is less than    approximately 5 nm in thickness. Hence the material can be described    as discreetly encapsulated.

The apparatus and process according to the present invention provide asimplified technique for the production and collection of sub-micron andnanometric powders. In a preferred embodiment, a transferred plasma arcis established between the arc tip of an elongate graphite electrode anda counter electrode formed as part of a graphite reactor crucible.

The apparatus according to the present invention may operate withoutusing any water-cooled elements inside the plasma reactor and allowsreplenishment of feed material without stopping the reactor.

The reactivity of sub-micron and nano-metric metals, such as aluminium,presents an operational risk if there is a likely-hood of contact withwater, reactive liquids, or reactive gases such as air and oxygen. Thepassivation stage described herein renders the powdered material moresuitable for transporting.

1. A plasma arc reactor for producing a powder from a solid feedmaterial, the reactor comprising: (a) a first electrode, (b) a secondelectrode spaced apart from the first electrode by a distance sufficientto achieve a plasma arc therebetween, and (c) a plasma gas disposedwithin the space between the first and second electrodes, wherein thefirst electrode has a channel running therethrough, an outlet of thechannel exiting into the space between the first and second electrodes,whereby a length of the solid feed material is fed through the channelto exit therefrom via the outlet into the space between the first andsecond electrodes.
 2. A plasma arc reactor as claimed in claim 1,further comprising solid feed material in the form of a wire and meansfor conveying said wire to the first electrode.
 3. A plasma arc reactoras claimed in claim 1, further comprising solid feed material in theform of a wire and a container or holder for said wire.
 4. A plasma arcreactor as claimed in claim 3, further comprising means for conveyingsaid wire from the container or holder to the first electrode.
 5. Aplasma arc reactor as claimed in claim 1, wherein the first electrode ismoveable with respect to the second electrode from a first position atwhich an arc portion thereof contacts with an arc portion of the secondelectrode to a second position at which said arc portions are spacedapart from each other by a distance sufficient to achieve a plasma arctherebetween.
 6. A plasma arc reactor as claimed in claim 1, wherein thefirst electrode is a hollow elongate member whose inner surface definesa closed channel, the elongate member terminating at an arc tip whichopposes the second electrode, wherein the outlet of the closed channelis disposed at or adjacent to the arc tip.
 7. A plasma arc reactor asclaimed in claim 1, wherein an arc portion of one of the first andsecond electrodes is formed from graphite.
 8. A plasma arc reactor asclaimed in claim 1, further comprising cooling means for cooling andcondensing solid feed material which, in use, has been vaporised in theplasma are between the first and second electrodes.
 9. A plasma arcreactor as claimed in claim 1, further comprising a cooling gas forcooling and condensing solid feed material which, in use, has beenvaporised in the plasma arc between the first and second electrodes. 10.A plasma arc reactor as claimed in claim 9, wherein the second electrodecomprises a graphite vessel having a surface adapted to direct vaporisedsolid material downstream to a cooling zone to be cooled, in use, by thecooling gas.
 11. A plasma arc reactor as claimed in claim 1, furthercomprising a collection zone for collecting powdered material.
 12. Aplasma arc reactor as claimed in claim 1, wherein the channel in thefirst electrode introduces the plasma gas into the space between thefirst and second electrodes.
 13. A plasma arc reactor as claimed inclaim 1, further comprising a DC or AC power source for generating aplasma arc in the space between the first and second electrodes.
 14. Aprocess for producing a powder from a solid feed material, comprisingthe steps of: (a) introducing a plasma gas into the space between firstand second electrodes of a plasma arc reactor, (b) generating a plasmaarc in the space between the first and second electrodes, (c) feeding alength of a solid material through a channel in the first electrode toexit via an outlet thereof into the plasma arc, whereby the solid feedmaterial is vaporised, and (d) cooling the vaporised material tocondense a powder.
 15. A process as claimed in claim 14, wherein thesolid feed material comprises a metal or alloy.
 16. A process as claimedin claim 15, wherein the solid feed material is aluminium or an alloythereof.
 17. A process as claimed in claim 14, wherein the plasma gascomprises an inert gas.
 18. A process as claimed in claim 17, whereinthe plasma gas comprises helium and/or argon.
 19. A process as claimedin claim 14, further comprising injecting the plasma gas through thechannel of the first electrode to exit therefrom into the space betweenthe first and second electrodes.
 20. A process as claimed in claim 19,wherein the plasma gas and solid feed material exit the first electrodevia a common outlet.
 21. A process as claimed in claim 19, wherein theplasma gas and solid feed material enter the channel in the firstelectrode via a common inlet.
 22. A process as claimed in claim 14,wherein at least some cooling of the vaporised material is achievedusing an inert gas stream.
 23. A process as claimed in claim 14, whereinat least some cooling of the vaporised material is achieved using areactive gas stream.
 24. A process as claimed in claim 14, furthercomprising oxidizing the surface of the powder using a passivating gasstream.
 25. A process as claimed in claim 24, wherein the passivatinggas comprises an oxygen-containing gas.
 26. A process as claimed inclaim 25, wherein the oxygen-containing gas comprises from 95 to 99 vol.% of an inert gas and from 1 to 5 vol. % of oxygen.
 27. A process asclaimed in claim 26, wherein the oxygen-containing gas comprisesapproximately 98 vol. % of an inert gas and approximately 2 vol. % ofoxygen.
 28. A process as claimed in claim 14, wherein the powdercomprises particles substantially all of which have a diameter of lessthan 200 mn.
 29. A process as claimed in claim 14, wherein the reactoris pre-heated to a temperature of from 2000 to 2500° C. prior tovaporising the solid feed material.
 30. A process as claimed in claim14, wherein the pressure in the reactor is maintained at a level aboveatmospheric pressure.
 31. A process as claimed in claim 14, wherein thepowder comprises particles having a core comprising or consistingessentially of aluminium and a surface layer comprising or consistingessentially of aluminium oxide.
 32. A process as claimed in claim 31,wherein the aluminium oxide surface layer has a thickness of ≦10 nm. 33.A process as claimed in claim 31, wherein substantially all of theparticles have a diameter of ≦200 nm.
 34. A process as claimed in claim31, wherein the average particle diameter lies in the range of from 50to 150 nm.
 35. A process as claimed in claim 31, wherein the particulatematerial has a specific surface area in the range of from 15 to 40m²g⁻¹.
 36. A process as claimed in claim 31, wherein the particles havea mono-crystalline core.
 37. A process for producing a passivatedaluminium powder from a solid feed material of aluminium wire, whichprocess comprises: a) supplying inert gas to a plasma arc reactor andgenerating an inert plasma in the reactor, b) feeding aluminium wireinto the inert gas plasma, whereby the aluminium is vaporized, c)cooling the vaporized aluminium with an inert gas to condense a powderof aluminium, and d) oxidizing the surface of the powder of aluminiumwith a passivating gas, and wherein the reactor comprises i) a firstelectrode having a channel running therethrough, and ii) a secondelectrode spaced apart from the first electrode by a distance sufficientto achieve a plasma arc therebetween, an outlet of the channel of thefirst electrode exiting into the space between the first and secondelectrodes, wherein said step of feeding aluminium wire into the inertgas plasma comprises passing the aluminium wire through the channel toexit therefrom via the outlet into the space between the first andsecond electrodes.